This invention relates to the fabrication of photodetectors, and, more particularly, to the fabrication of an InGaAs PIN photodetector having high optical-power handling capability with good linearity of conversion to an electrical signal.
A photodetector is a device that converts incident light into an electrical signal. A light beam is directed onto the photodetector, and the electrical output signal of the photodetector is a measure of the incident energy of the light beam. Photodetectors are available for both visible and infrared light energy.
Photodetectors are used in a wide variety of applications. For some, the photodetector need only detect the presence of light, and its other component capabilities are not important.
In other applications, however, the photodetector is used in a signal processing or handling system. The photodetector may act as a part of a signal handling system to convert incident light to an electrical signal for transmission or processing. The photodetector must therefore be capable of handling the desired input power of the light beam, and achieving the conversion to an electrical signal with good linearity, good frequency range, and low distortion.
A well known photodetector for the near infrared light range is the InGaAs PIN diode. In one version, this diode has a p+ doped InGaAs layer and an n+ InP layer, on either side of an undoped InGaAs layer, with this structure supported on a light-transparent InP substrate. Light incident on the front side of the substrate produces a voltage between the p+ doped InGaAs layer and the n+ InP layer, which voltage is generally proportional to the intensity of the incident light.
While operable, this InGaAs PIN diode has some shortcomings for particular applications, such as CATV analog transmission systems using RF and microwave antenna networks. For such systems to achieve their best performance, the maximum light intensity must be in the range of greater than 10 milliwatts (mW) of optical signal strength and the light-to-electrical signal conversion must have good linearity. Semiconductor lasers capable of producing an optical signal output in the range of a few tens of milliwatts are now available. However, most existing InGaAs PIN photodiodes are limited to about 2 mW of incident optical signal strength. The systems using the available InGaAs PIN photodiodes therefore cannot take advantage of the capabilities of the higher-power semiconductor lasers. Accordingly, the available photodetectors limit the performance of these systems.
There is a need for an improved photodetector which can handle high incident optical intensities and convert them to electrical signals with good linearity. The present invention fulfills this need, and further provides related advantages.